The nature of attention and its role in perception and cognition are fundamental questions in the study of the higher functions of the human brain. This proposal describes a research plan in which the best available neurophysiological and behavioral methods are used to address these issues directly. Two sets of neurophysiological procedures, pupillometric measurement of task-induced variations in attention, and event-related brain potential assessment of stages of stimulus processing, are developed and utilized in combination with the quantitative psychophysical methods of signal detection theory in a series of eleven studies. The program of studies will help clarify the nature and role of attentional processing in signal detection and identification. It is anticipated that some surprising and counterintuitive findings will be encountered, such results having already appeared in preliminary work. Thus, the results of these studies will have major implications for theories of perception. It is also likely that these studies of the neurophysiology of attention and perception will lead to important conclusions about the general properties of attentional and other higher cortical functions of the human nervous system. The results of the research will be of relevance to a number of scientific discliplines, including human neurophysiology, psychophysics, cognitive psychology and sensory physiology.